Your deck is very similar to mine. I've been toying with getting rid of my Kird Apes for a set of Helix or Boros Charm and going towards more what you have.

Two questions though: Why not Birds over Heirarch for the all important black/whatever color you need? I'd love to hear about your thoughts on the sideboard.

When it came down to it I usually focused more on black from the land. Also the heirarch is more so for the turn 2 Giest you still get from Birds. However, Bird is a dead draw closing to late game. The side board was a stupid thing I came up with when I wanted to jam gifts ungiven. I love the card but it's not seeing much play and I can just switch to another archtype especially against burn or Affinity. Martyr proc was a serious problem. The smiter is really the flex point in my opinion. I might go 1 bird with 4 hierarch if anything. Plus the dream is as always turn 1 hierarch turn 2 giest turn 3 rhino. Chances of that happening are low some cards are meant to change. Won't deny this deck has evolved from aggro to more aggro/midrange. Something I am a bit uncomfortable to admit to.